1. Field of the Invention
This invention relates to a method of producing an easily openable container closure having a shell and sealing member. More specifically, this invention relates to a method of producing an easily openable container closure comprising (1) a shell of a unitary structure including a top and a cylindrical skirt, said top having an annular top surface portion leading to the upper end of the skirt and a pull ring connected to a part of the inner peripheral edge of the annular top surface portion through a linking piece, and the annular top surface portion and the skirt having a pair of breaking weakened lines formed along the phantom lines extending from both side edges of the linking piece, and (2) a sealing member positioned within the shell for closing an opening present in the top surface of the shell.
2. Description of the Prior Art
French Pat. No. 2,231,578 and U.S. Pat. No. 3,596,790 disclose an easily openable container closure of the above-mentioned structure for use in closing the mouth of a container such as a bottle or can. Because such a container closure can be easily opened without using such an instrument as an opener, and can be produced at a lower cost than the other types of easily openable container closures disclosed, for example, in Japanese Patent Publication No. 26107/76 and Japanese Laid-Open Patent Publication No. 29073/72, its use has been strongly desired recently in place of ordinary crown caps or the easily openable container closures of the types disclosed in the above Japanese Patent documents.
The known easily openable container closures disclosed in the French and American Patents pose various problems because, for example, the pull ring provided in the shell tends to project upward beyond the upper end of the skirt of the shell and the upper surface of the annular top surface portion.
In the container closure disclosed in French Pat. No. 2,231,578, the annular top surface portion of the shell extends inwardly and substantially horizontally from the upper end of the skirt, and from the inner peripheral edge of the annular top surface portion, the linking piece and the pull ring extend inwardly and substantially horizontally, whereby the upper end of the skirt, the annular top surface portion, the linking piece and the pull ring are positioned substantially on the same plane. However, because an inwardly directed force is exerted on the lower end portion of the skirt at the time of mounting a container closure on the mouth of a bottle or the like, the annular top surface portion and the linking piece are inclined upwardly in the inward direction, and as a result, the pull ring tends to projects upwardly beyond the annular top surface portion. Pull rings having such a tendency collide with various objects during the transportation of bottles provided with closures or during the transfer of such bottles in automatic vendor machines. This results in undue forces on the pull rings, and the shells may be accidentally broken along a pair of weakened lines. Or the transfer of bottles in automatic vending machines is obstructed, and the operation of the automatic vendors becomes temporarily out of order.
In the container closure disclosed in U.S. Pat. No. 3,596,790, in an attempt to solve the problems associated with the container closure disclosed in the above-cited French Patent, the linking piece connecting the annular top surface portion and the pull ring of the shell to each other is caused to extend downwardly in an inclined manner, instead of inwardly and substantially horizontally, from a part of the inner peripheral edge of the annular top surface portion, whereby the pull ring extending inwardly from the inside end of the linking piece is located at a lower position than the annular top surface portion. In a container closure of such a construction, too, the annular top surface portion is formed only of a portion extending substantially horizontally, and therefore, the rigidity of the annular top surface portion is not sufficient. When a force is exerted on the lower end portion of the skirt of the shell to deform it inwardly at the time of mounting the container closure around the mouth of a container such as a bottle, the annular top surface portion and the linking piece are displaced inclinedly upwardly in the inward direction and in the direction of the inside diameter. Owing to this, the pull ring is displaced upwardly, and a part of it tends to be located at a higher position than the annular top surface portion. Furthermore, since the annular top surface portion does not have sufficient rigidity, it is likely that when the container closure is mounted on the mouth of a container, the annular top surface portion will be deformed, or a pair of breaking weakened lines formed in the annular top surface portion will be broken at an early stage. Moreover, even after mounting, the annular top surface portion may be deformed easily upon the application of impact, resulting in the reduced ability to seal the container.